Targeted by his rivals for weeks, the GOP front-runner fired back in a campaign debate: “This ain’t beanbag … we’re going to describe the differences between us.” It was the most intense exchange of a weekend debate double header, run-up to Tuesday’s New Hampshire primary.

Romney, the former Massachusetts governor, won the Iowa caucuses last Tuesday by a scant eight votes over former Pennsylvania Sen. Rick Santorum but is so far ahead in New Hampshire polls that his rivals have virtually conceded he will win.

South Carolina comes next, on Jan. 21, the first Southern state to hold a primary. While it is the contest where Gingrich, Santorum and the rest of Romney’s rivals face an urgent need to slow his candidacy, Romney pointedly noted that he has been endorsed by that state’s governor, Nikki Haley.

Santorum finished second in Iowa, followed by Texas Rep. Ron Paul, with Gingrich fourth, Texas Gov. Rick Perry fifth and Minnesota Rep. Michele Bachmann in last place. She has since quit the race. Former Utah Gov. Jon Huntsman skipped Iowa in hopes of a breakout showing in New Hampshire.

The GOP contenders faced off Sunday for the second time in less than 12 hours, following their debate Saturday night in nearby Manchester.

Though aggressive in assailing Romney, Gingrich hedged when he was confronted with one of his own campaign leaflets declaring Romney to be unelectable against President Barack Obama. “I think he’ll have a very hard time getting elected,” was as far as he would go.

It wasn’t all testy. Perry drew laughter as well as applause when he said that federal bureaucrats would experience pain as a result of his plans to cut spending, especially those in the departments of education, commerce and energy. That was a reference to his gaffe in an earlier debate when he couldn’t recall the name of the third of the Cabinet-level agencies he has proposed eliminating.

Indicating he’d taken offense on another matter, Huntsman, who was Obama’s ambassador to China before quitting to run for the White House, returned to a comment Romney had made the night before. Romney said then that the rest of the GOP hopefuls had been trying to oppose the administration’s policies while Huntsman was advancing them.

“And I just want to remind the people here in New Hampshire and throughout the United States, he criticized me while he was out raising money for serving my country in China, yes, under a Democrat, like my two sons are doing in the United States Navy. They’re not asking what political affiliation the president is. “

As was the case Saturday night, Romney sought to shrug off the attacks from his rivals on the debate stage and worked to turn the focus onto Obama, whom he accused of presiding over an unnecessarily long recovery from recession.

Obama has been “anti-investment, anti-jobs and anti-business,” he said.

But Gingrich was more aggressive than he had been Saturday night, his attacks serving as bookends to the 90-minute event.

The former speaker briefly led in the Iowa and national polls before the caucuses, before his surge was blunted by a series of ads aired by a so-called super PAC that is operated by former Romney aides and allies.

Gingrich has complained bitterly that the attacks were false, but he was asked on Sunday about a similar organization set up by his own supporters. It is intent on criticizing Romney for having run an investment firm that cost workers their jobs when it took over their companies.

Asked if he was being consistent, Gingrich said, “I’m consistent because I think you ought to have fact-based campaigns.” He demanded Romney say whether the attacks against him were true.

Romney replied, “I haven’t seen them, and as you know, under the law, I can’t direct the ads. If there’s anything in the ads that are wrong, I hope they take it out.”

Yet moments after saying he hadn’t seen the commercials, he recited the charges they made and said they were accurate — that Gingrich had been forced to resign as speaker, that he had once talked of finding common ground with House Democrats on climate change and that he had called a House Republican proposal to overhaul Medicare “right wing social engineering.”

Gingrich said he was glad “he has said weeks later if they’re wrong they should take them down.”

Almost 90 minutes earlier, Gingrich called Romney a “relatively timid Massachusetts moderate” whose state ranked fourth from the bottom in job creation when he was governor.

Romney said he had created more jobs in one state than Obama had in the entire country, adding that it was important to replace “a lifetime politician” like the president with a different type of leader.

Santorum, too, took a swipe at Romney, asking why he hadn’t sought re-election as governor after one term.

“Why did you bail out? And the bottom line is, I go fight the fight,” Santorum said, referring to his time in Congress in the House of Representatives from a blue-collar district.

Romney jabbed back with a reference to Santorum’s lucrative career in the six years since he lost a re-election campaign in 2006.

“I long for the day when instead of having people to go to Washington for 20 to 30 years, will get elected and then when they lose office, they stay there and make money as lobbyists or conducting their businesses.

“I think it stinks,” Romney said.

Moments later, Gingrich appeared irked and accused Romney of using more than his allotted time to respond.

“I realize the red light doesn’t mean anything to you because you’re the front-runner.”

“Could we drop a little bit of the pious baloney. The fact is you ran in ‘94 and lost (to Ted Kennedy). … You were running for president while you were governor. … You’ve been running consistently for years.”